1. Field of the Invention
The present invention relates to a light emitting semiconductor device, more particularly, to a high output light emitting semiconductor device made of a III-V mixed compound semiconductor crystal having a p-n junction, and to a method for making the same.
2. Description of the Prior Art
Various types of light emitting semiconductor devices are already known. They are often made of a III-V mixed compound semiconductor crystal, such as GaAs.sub.1.sub.-x P.sub.x and Ga.sub.1.sub.-x Al.sub.x As, having a dome or hemispherical shape so that light generated at a p-n junction, which is formed mostly at the center of the planar surface of the hemisphere, can be emitted outside the crystal without total reflection at the surface of the crystal. In one of these hemispherical dome type geometry devices, the major part of the crystal is of n-conductivity type, and a p-conductivity type region is formed by diffusion at the central portion of the hemispherical crystal. One electrode is formed on the planar surface of the p-conductivity type region and another electrode, in a circular band shape, is also formed on the planar surface of the n-conductivity type region near the circular edge of this surface. The band gap is narrowest at the planar surface and becomes wider as the distance from this surface increases. This distribution of band gap width is most easily obtained by using segregation during crystal growth. Owing to this distribution of band gap width, light generated at the p-n junction formed between the p and n conductivity type regions is hardly absorbed in the crystal and is externally emitted with high efficiency.
In another type of device, a p-conductivity type layer is epitaxially grown on the surface of an n conductivity type III-V mixed compound semiconductor substrate which has a narrower band gap than the surface of opposite side. A circular portion including a part of this layer, which is slightly thicker than the p conductivity type layer, is removed by mesa-etching so that a p-n junction is delimited by this mesa-etching. The crystal is shaped in a dome or hemisphere, and two electrodes are formed on the n- and p-conductivity type layers as described for the type where the surface on which electrodes are provided is planar, and the p conductivity type region is formed by diffusion.
The first type of light emitting device, where the p-conductivity type region formed by selectively diffusing impurities is used directly as one of the two regions forming a p-n junction, has the disadvantage that carrier mobility is small and light emission efficiency is low due to the coexistenance of donars, such as Te, and acceptors, such as Zn, in the p-conductivity type region.
The second type of light emitting device, where the p-n junction is formed by epitaxial growth and delimited by mesa-etching, has the drawback that the diode must be mounted face-down on an auxiliary mounting device having two surfaces, the difference in height of which is equal to the height of the mesa, in order to connect the positive and negative electrodes to their respective external leads.
This complicates the process and lowers the yield rate of fabrication. Furthermore, this type of mounting increases both the thermal and electrical resistances. For this type of light emitting device, the electrical contact resistance at the positive and negative electrodes and the series resistance component of the p-conductivity type Ga.sub.1.sub.-x Al.sub.x As layer cannot be significantly reduced, because the majority carrier concentration in the p-conductivity type layer and that in the n conductivity type layer are preferably 1 - 2 .times. 10.sup.18 cm.sup.-.sup.3, taking the injection efficiency and crystallographical structure into account. Since the external quantum efficiency of these high output power light emitting devices is ordinarily from 5 to 15%, high thermal and electrical resistances are serious drawbacks of this type of light emitting device.